6 



Journal of Applied Microscopy. 



effect, as the cover, oil, and front lens of 

 the objective form a practically homo- 

 geneous mass. 



Remedy. — Use cover glasses varying little 

 from the generally accepted standard, i. e., 

 16 mm. 



(2.) Improper Use of Abbe Condenser. — 

 A superabundance of light obliterates all 

 detail and creates a glare uncomfortable 

 to the eye. 



Remedy — Except in the case of oil immer- 

 sion objectives on stained bacteria, when the 

 condenser is used with the full opening, the 

 aperture of the condenser should always be 

 reduced to a trifle less than the angular aper- 

 ture of the objective. 



In histological, and other dense objects, 

 reduce the Iris diapliragm to about one-half 

 the opening of rear lens in objective. 



On diatoms reduce the diaphragm to about 

 two-thirds the rear lens in objective. 



The diameter of rear lens may be judged by 

 removing the eyepiece. 



Unclean Lenses. 



If in either objective or eyepiece 

 the lenses are not clean, the defini- 

 tion may be seriously reduced or 

 destroyed. Finger marks upon the front 

 lens of objective, or upon eyepiece lenses, 

 dust which in time may settle upon rear 

 lens of objective or on eye lens, film 

 which forms upon one or the other lens, 

 due occasionally to the fact that glass 

 is hygroscopic, but generally to the ex- 

 halation from the interior finish of the 

 mountings; and, finally, in immersion 

 objectives, because the front lens is not 

 properly cleaned, or oil has leaked onto 

 its rear surface, or air hubbies have 

 formed in the oil between the cover 

 glass and front lens. The latter two 

 causes may totally destroy all definition, 

 no matter how excellent the objective is 

 or may have been. 



Remedy. — Keep all lenses scrupulously 

 clean. For cleaning, use well-washed linen 

 {an old handkerchief) or Japanese lens paper . 



Eyepieces. — ^To find impurities, revolve 

 the eyepieces during observation; 

 breathe upon the lenses, and wipe gently 

 with a circular motion and blow off any 

 particles w^hich may adhere. 



Dry Objectives. — Clean front lens as 

 above. To examine rear and interior 

 lenses use a two-inch magnifier, looking 

 through the rear. Remove dust from 

 rear lens with a camel's hair brush. 



Oil-Immersion Objectives. — Invariably 

 clean front lens after use with a moist- 

 ened linen or paper, and wipe dry. 



In applying oil examine the front of 

 objective with a magnifier, and if there 

 are any air bubbles remove with a point- 

 ed quill, or remove oil entirely and apply 

 a fresh quantity. 



Spherical Aberration. 



This may be caused by: 



(1.) Covers varying from normal thick- 

 ness: treated under "Inferior Definition" 

 with the same remedy. 



(2.) Tube length varying from the nor- 

 mal length to which objectives are cor- 

 rected. 



Remedy. — Cover glass of standard thick- 

 ness; tube length according to correction of 

 objective. 



The pernicious influence of covers 

 varying from normal thickness is thus 

 again emphasized, and can not be too 

 strongly impressed. 



Reduced Illumination. 



This may be caused by: 



(1.) Higher-power eyepiece. There is 

 a proportionate increase in loss of light 

 as the power of eyepiece is increased, or, 



(2.) Between two objectives of differ- 

 ent power, the higher will give less 

 illumination. 



(3.) Small aperture and diaphragm, 

 either with or without condenser. 



(t.) Thick section or dense object. 



(5.) Mirror tilted so as not to give 

 complete illumination or to cause the re- 

 flection of an obstruction, such as a win- 

 dow sash, to fall in the optical axis. 

 This latter feature may not only affect 

 the amount but the character of the 

 illumination, as oblique illumination may 

 be obtained even when the mirror is in 

 the central position by suitably tilting 

 it. 



Short Working Distance. 



This may be created by: 



(1.) The use of thicker cover glasses 

 than the normal; in abnormally thick 

 ones, when the air space between the ob- 

 jective and cover is taken up by the 

 increased thickness of the latter, it may 

 even prevent reaching a focus on the 

 object. 



(2.) Thick sections or objects may re- 

 duce working distance to such an ex- 

 tent as to prevent focusing through the 

 cover. 



(3.) If the cover is not in direct con- 

 tact with the object, it may be and often 

 is reduced. 



In conclusion, I would add that there 

 remains still one more factor, that of 

 accidental injury to the objective by fall- 

 ing or coming in violent contact with 

 the cover while focusing, against which 

 every precaution should be used. The 

 front lenses are the most exposed and 

 at the same time the most delicate, and . 

 contact against a hard surface is likely 

 to displace or injure them. Particular- 

 ly is this the case in the oil-immersion 



